1. Field of the Invention
This invention relates to a connector for providing a "hot pluggable" connection between an electronic circuit board and one or more printed circuit cards carried by the board.
2. Description of the Related Art
A highly desirable feature in a system containing electronic circuit boards is a "concurrent maintenance" capability, whereby a part of the system may be serviced simultaneously with the normal operation of the remainder of the system. This has the obvious advantage that the entire system need not be deactivated simply to service or replace the defective part. To provide such a concurrent maintenance capability, systems have been devised for allowing for the "hot plugging" cards into boards while the system is powered up and in normal operation. Generally, such systems involve providing a current-limiting circuit on the card, through which the card load is initially charged before supplying the card load with power directly. This has been implemented, for example, using field-effect transistors (FETs) such as metal oxide semiconductor (MOS) FETs, in conjunction with sequential connectors in which certain pins are staggered so as to make or break an electrical connection before other pins of the connector. In such systems, the connection sequence is such as first to connect the card ground and board ground, then connect the voltage supply from the board to the current limiter on the card, and finally to connect the voltage supply to the card load directly.
While such systems are capable of providing hot pluggability and concurrent maintenance, several problems remain. First, there is the obvious expense of providing a separate current-limiting circuit on each card which may plug into a board. Second, such systems are susceptible to high-frequency noise resulting from parasitic oscillations occurring when the drain pin of the board is joined to the card. This connection typically produces a sudden inrush of current into the card to charge the MOSFET's parametric capacitance, typically about 3,000 pf. Still another problem associated with systems of the prior art is the electrostatic discharge (ESD) that often results when a card is mated with a board. In the past, this problem has required the use of conductive card holders, which provide a shunt path between the board ground and the card ground, bypassing sensitive electronic components.